A typical traction elevator includes a car and counterweight connected together by a plurality of ropes. The ropes extend over a traction sheave that is engaged with a drive machine. As a result of traction between the ropes and sheave, rotation of the sheave by the drive machine causes the car and counterweight to move in opposite directions through the hoistway. The magnitude of the traction between the ropes and traction sheave is dependant upon the friction between the ropes and sheave, the length of contact between the ropes and sheave, and the tension in the ropes.
The friction between the ropes and sheave may be increased by changing the contour of the groove of the sheave. This, however, may lead to increased wear of the sheave and ropes. Another possibility is to place a liner in the groove to enhance the friction between the ropes and sheave. This configuration has been successfully used to increase traction while minimizing the wear of the ropes and the sheave. The liners wear, however, and require replacement.
The length of contact between the ropes and sheave may be varied to optimize the amount of traction. In a single wrap roping configuration, which is the most common type, the length of contact is less than 180 degrees. Various methods to increase the length of contact have been used, such as long-wrap configurations and double-wrap configurations. Both of these configurations increase the length of contact to beyond 180 degrees. A drawback to increasing the length of contact, however, is the increased amount wear of the rope and sheave.
The tension in the ropes is dependant upon the weight of the car and counterweight. The heavier the car, counterweight and ropes, the more traction is generated. Unfortunately, the traction forces generated on the ropes and sheave are not uniform. For the single wrap configuration, the maximum traction force occurs at the midpoint between the take-up point and the take-off point for the ropes, with the minimum traction occurring at those points. This non-uniform distribution of loads may result in peak loads that cause damage or excessive wear of the ropes and sheave.
In addition, heavier components also increase the load on the drive machine. In general, the trend is toward the use of lightweight materials to produce lightweight components and thereby reduce the loads and the size of the drive machines, brakes, etc. The reduction in weight is limited, however, by the need to generate sufficient traction to drive the car and counterweight.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop traction elevator systems that minimize wear of the ropes and traction sheaves and permit the weight of the car and counterweight to be minimized.